Pentobarbital alters compensatory neural and humoral mechanisms in response to hemorrhage

Abstract

The effects of hemorrhage (1 ml/s) to a total blood loss of 30 ml/kg were examined in the same chronically instrumented dogs on separate days, conscious or anesthetized with pentobarbital sodium (25 mg/kg), and with ventilation controlled to maintain arterial blood gases at physiological levels. In conscious dogs, 30 ml/kg of blood loss reduced mean arterial pressure by 28 .+-. 7% and cardiac output by 59 .+-. 4%, while total peripheral and iliac vascular resistances had risen by 81 .+-. 17 and 142 .+-. 36%, respectively. Anesthesia did not affect control levels of these hemodynamic parameters significantly. At 30 ml/kg of blood loss, the reductions in mean arterial pressure (55 .+-. 5%) and cardiac output (73 .+-. 3%) were more profound (P < 0.01), whereas the increases in iliac vascular resistance (42 .+-. 15%) were blunted (P < 0.01). Pentobarbital increased control values of plasma renin activity and depressed those for catecholamines. In anesthetized dogs, plasma renin activity began to increase at 5 ml/kg of blood loss and rose by 13 .+-. 1.2 ng angiotensin I .cntdot. ml-1 .cntdot. h-1 at 30 ml/kg of hemorrhage. In conscious dogs plasma renin activity rose by only 4.4 .+-. 0.8 ng angiotensin I .cntdot. ml-1 .cntdot. h-1 at 30 ml/kg of hemorrhage. A different pattern was observed for plasma catecholamines; in conscious dogs, at 30 ml/kg of hemorrhage plasma norepinephrine increased by 668 .+-. 154 pg/ml and epinephrine by 901 .+-. 205 pg/ml, whereas in anesthetized dogs hemorrhage increased plasma norepinephrine by only 209 .+-. 15 pg/ml and plasma epinephrine failed to rise. Pentobarbital anesthesia modifies strikingly the ability to maintain arterial pressure with hemorrhage. This was associated with a depressed sympathoadrenal response, but the renin-angiotensin system was not depressed. Pentobarbital anesthesia does not appear to elevate sympathetic tone in the healthy, trained, chronically instrumented animal.